Bottom Line:
This is because most extinctions ( approximately 70%) occur at high trophic levels (top predators and other carnivores), while most invasions are by species from lower trophic levels (70% macroplanktivores, deposit feeders, and detritivores).The consequences of the simultaneous loss of diversity at top trophic levels and gain at lower trophic levels is largely unknown.However, current research suggests that a better understanding of how such simultaneous changes in diversity can impact ecosystem function will be required to manage coastal ecosystems and forecast future changes.

Affiliation: Center for Population Biology, University of California, Davis, California, United States of America. jebyrnes@ucdavis.edu

ABSTRACTThe biodiversity of ecosystems worldwide is changing because of species loss due to human-caused extinctions and species gain through intentional and accidental introductions. Here we show that the combined effect of these two processes is altering the trophic structure of food webs in coastal marine systems. This is because most extinctions ( approximately 70%) occur at high trophic levels (top predators and other carnivores), while most invasions are by species from lower trophic levels (70% macroplanktivores, deposit feeders, and detritivores). These opposing changes thus alter the shape of marine food webs from a trophic pyramid capped by a diverse array of predators and consumers to a shorter, squatter configuration dominated by filter feeders and scavengers. The consequences of the simultaneous loss of diversity at top trophic levels and gain at lower trophic levels is largely unknown. However, current research suggests that a better understanding of how such simultaneous changes in diversity can impact ecosystem function will be required to manage coastal ecosystems and forecast future changes.

pone-0000295-g003: Trophic skew in the Wadden Sea in (A) uninvaded intact communities, (B) exotic species that have successfully established and (C) species that have gone extinct.Data presented as the percentage of species in each trophic group. Colors indicate trophic level (white = 1, light grey = 2, dark grey = 3, black = 4). Patterns match those in the larger global/regional data set (Fig. 2), with extinctions and invasions occurring within different trophic groups, and neither matching the natural trophic distribution of species.

Mentions:
These patterns of trophic skew from invasions and extinctions remain intact when analyses are restricted to spatially congruent subsets of the data for the Wadden Sea (Fig. 3, χ210 = 41.47 p<0.0001). Both invasions and extinctions in the Wadden Sea differ in their pattern of species distributions among trophic levels from the trophic distribution of our reconstructed pre-disturbance species list (Fig. 3, Invasions χ210 = 24.79 p = 0.0058, Extinctions χ210 = 57.93 p<0.0001). The pre-invasion and extinction species list shows a classic “pyramid” in shape with decreasing numbers of species with increasing trophic level (Fig. 4A). The differential distribution of invasions and extinctions among trophic groups have already caused measurable changes in the relative distribution of species among trophic levels, even with invasions and extinctions each comprising only 5.1% of the total species (Fig 4D). While species richness has remained nearly the same, there are now 14% fewer predator species and 8.6% more primary consumer species.

pone-0000295-g003: Trophic skew in the Wadden Sea in (A) uninvaded intact communities, (B) exotic species that have successfully established and (C) species that have gone extinct.Data presented as the percentage of species in each trophic group. Colors indicate trophic level (white = 1, light grey = 2, dark grey = 3, black = 4). Patterns match those in the larger global/regional data set (Fig. 2), with extinctions and invasions occurring within different trophic groups, and neither matching the natural trophic distribution of species.

Mentions:
These patterns of trophic skew from invasions and extinctions remain intact when analyses are restricted to spatially congruent subsets of the data for the Wadden Sea (Fig. 3, χ210 = 41.47 p<0.0001). Both invasions and extinctions in the Wadden Sea differ in their pattern of species distributions among trophic levels from the trophic distribution of our reconstructed pre-disturbance species list (Fig. 3, Invasions χ210 = 24.79 p = 0.0058, Extinctions χ210 = 57.93 p<0.0001). The pre-invasion and extinction species list shows a classic “pyramid” in shape with decreasing numbers of species with increasing trophic level (Fig. 4A). The differential distribution of invasions and extinctions among trophic groups have already caused measurable changes in the relative distribution of species among trophic levels, even with invasions and extinctions each comprising only 5.1% of the total species (Fig 4D). While species richness has remained nearly the same, there are now 14% fewer predator species and 8.6% more primary consumer species.

Bottom Line:
This is because most extinctions ( approximately 70%) occur at high trophic levels (top predators and other carnivores), while most invasions are by species from lower trophic levels (70% macroplanktivores, deposit feeders, and detritivores).The consequences of the simultaneous loss of diversity at top trophic levels and gain at lower trophic levels is largely unknown.However, current research suggests that a better understanding of how such simultaneous changes in diversity can impact ecosystem function will be required to manage coastal ecosystems and forecast future changes.

Affiliation:
Center for Population Biology, University of California, Davis, California, United States of America. jebyrnes@ucdavis.edu

ABSTRACTThe biodiversity of ecosystems worldwide is changing because of species loss due to human-caused extinctions and species gain through intentional and accidental introductions. Here we show that the combined effect of these two processes is altering the trophic structure of food webs in coastal marine systems. This is because most extinctions ( approximately 70%) occur at high trophic levels (top predators and other carnivores), while most invasions are by species from lower trophic levels (70% macroplanktivores, deposit feeders, and detritivores). These opposing changes thus alter the shape of marine food webs from a trophic pyramid capped by a diverse array of predators and consumers to a shorter, squatter configuration dominated by filter feeders and scavengers. The consequences of the simultaneous loss of diversity at top trophic levels and gain at lower trophic levels is largely unknown. However, current research suggests that a better understanding of how such simultaneous changes in diversity can impact ecosystem function will be required to manage coastal ecosystems and forecast future changes.